Heretofore, a water tube boiler has, on the whole, been composed of a combustion chamber made up of furnace wall water tubes and of water tubes forming a convective heating zone, i.e. the water tubes are arranged so as to surround the interior of the combustion chamber in the furnace, and a large number of water tubes are disposed in a dense arrangement downstream thereof so as to form a convective heating zone. Therefore, although the boilers have a size largely determined by the space of the combustion chamber, the heating surface of the water tubes, the numbers and the weight of water tubes, and hence, the cost of the boilers, nonetheless depend largely on the water wall tube heating bank set up in the back part (downstream) of the combustion chamber. The boilers heretofore in use have been such boilers as described above, and it has been desired to obtain a high degree of efficiency of heat transfer to the water tubes so as to reduce the cost of the boilers, and for the sake of the high effectiveness and miniaturization and reduction in weight of boiler, resulting in a corresponding reduction in the cost of the boiler. Accordingly, the design of water tubes heretofore in use for the reduction of boiler cost has been based on the view that it is better to dispose the water tubes as densely as possible in order that the water tubes assume a most compact arrangement.
On the other hand, the water tubes cannot be arranged too densely because of problems with the strength of the header and the drum. Accordingly the arrangement and the longitudinal pitch of the water tubes of the water tube bank heretofore have been determined from experience. Consequently, in the case in which the longitudinal pitch of the water tubes, i.e. the pitch in the direction of the gas flow, is L(mm) and the outer diameter of these water tubes is D(mm), L/D of the boilers heretofore in use has had a value of 1.5.
This value L/D=1.5 has been used commonly in the past as determined by experience without an evaluation of whether the value 1.5 represents good or poor parameters for the heat transfer coefficient, because a reasonably good method to evaluate this value of L/D=1.5 could not be found.